Printed circuit cards have been mounted to a chassis or heat sink with devices that are mounted to the printed circuit card and which, when actuated, retain the edge of the circuit card in a slot in a chassis, housing or other circuit card-retaining frame. The printed circuit cards have also been mounted to a frame or chassis utilizing edge connectors. Edge connectors, while protecting electrical contact, do not in general provide vibration or shock resistance, nor do they support thermal transfer to a heat sink.
Note that it is important not only to be able to connect the circuit board to other circuit boards or to wiring, it is also important to be able to secure the circuit board against vibration and more importantly to be able to dissipate the heat generated by components on the board so that as the circuit board components are operated, the heat is dissipated to prevent failure.
Nowhere is this more important than circuit card-carrying field programmable gate arrays (FPGAs), which are utilized in a wide variety of processing applications. One type of application includes a so-called jam cube, which is utilized to house a receiver and a transmitter driver. In this application, the jammer receives incoming signals and transmits appropriate jamming signals.
As electronics are available in smaller and smaller sizes that perform more and more functions, there is a tradeoff between power and size. As one decreases the size of the device, one still needs to dissipate the same amount of power when performing the functions that the device is supposed to perform. Additionally, decreasing size may result in increased power consumption that drives the operating temperature of the electronics up.
For instance, for typical commercially available FPGAs, their operating range is between 85 and 100° C. It will be appreciated that in some applications the FPGA modules may dissipate between 5 and 20 watts, with this power required to be dissipated into the environment. Oftentimes ambient air at the board is not enough to dissipate the heat generated by the components on the board. As can be readily seen, there is a correspondence between the amount of signal processing that is performed and required heat dissipation, with heat buildup being a significant failure mode for these circuits.
Moreover, the shrinking of electronic modules requires that the circuit boards be placed on tighter pitches such that there is a requirement to mount boards side by side on 3/10-inch pitches.
One previous method for mounting circuit cards involves a so-called wedge lock, which is attached to the edge of the circuit card. Such wedge locks are made in one embodiment by Calmark, which involves six moveable segments that are configured as trapezoids such that then the bar containing these segments is tightened, the wedges move outwardly and contact a wall of a slot in a heat sink, chassis or frame.
The problems with such wedge lock assemblies are two-fold, first having to do with the heat transfer efficiency and secondly, the size of the wedge lock assemblies that are mounted to the edges of the circuit cards.
It will of course be appreciated that each of the circuit cards must be provided with bars mounted to either end of the circuit card. For board re-spinning or replacement, if one wishes to switch out a circuit card, for instance, that is defective, one needs to mount these wedge lock assemblies to the ends of the new circuit card.
More important are the thermal barriers that are presented by these multi-part wedge lock structures. In their attempt to pull the heat out of the card and transfer it to the chassis, only a portion of the circuit card edge is thermally coupled to the heat sink by the wedge lock assembly.
It will be appreciated that the amount of heat that can be transferred from the card to the chassis is dependent upon the contact area between the card and the chassis. It also depends on the amount of pressure that can be exerted between the card and the chassis, with the higher the pressure the lower the resistance to heat flow.
Because in the multi-section wedge lock assembly wedges are spaced apart, typically only 50% of the available edge of a circuit card is thermally coupled to the chassis, heat sink or frame. Also, the contact pressure varies along the clamped edge of the card. Where there is contact, the pressure is high. Where there is no contact, the pressure is zero. Because of the way in which these wedge lock devices operate, there is thus increased the thermal resistance, which in essence fails to dissipate enough heat. It will be appreciated that one can directly relate contact area to the temperature of the components on the printed circuit. Since the thermal and physical contact area does not encompass the entire edge of the card, heat transfer and mechanical locking properties are not as robust as they could be.
In summary, the wedge lock type of card mounting assemblies present a barrier to heat transfer, since the temperature flow across each of the spaced-apart trapezoidal surfaces is only some percentage of the total area available.
Secondly and as mentioned above, mechanical retention of the card in the chassis is vitally important to minimize shock or vibration dislodgment of printed circuit boards. It is of paramount importance to provide a maximal amount of pressure to clamp the circuit board to the chassis, with any additional pressure also increasing the thermal transfer.
As before with the wedge lock assemblies, pressure is discontinuous along the edge of the card, with some areas under high pressure, while other areas under zero pressure. This discontinuity is believed to be more susceptible to circuit card damage under vibration at the interface of the high pressure/zero pressure zones than a continuously clamped circuit card would experience.
There are standard circuit board retainers such as manufactured by ZIF. These retainers have an elliptical cross-section cam that presses against a thin, flat spring that contacts the circuit board. However, this multi-part device offers a high-resistance heat transfer path due to the fact that heat has to go through a thin contacting blade, a cam and a thin wall.
In summary, there is a requirement to be able to mount and retain printed circuit cards in a chassis, heat sink or frame, first and foremost to maximize the thermal transfer between the card and the chassis. Secondly, there needs to be a mechanism for providing that the entire length of the edge of the card be clamped to the chassis, both for maximal heat transfer and for continuous pressure robust mounting. Thirdly, there is a requirement that the pitch of these boards be reduced so that multiple boards can be mounted in reduced-size modules. Lastly, prior art devices expand and move away from the circuit card as they are tightened. It would be preferable to produce a clamping device that moves towards the card, making tighter card spacing possible.
Additionally, there is a requirement to able to clamp the board to the chassis with easy access to tightening screws and, most importantly, to provide a universal mounting system that can accommodate any configuration of circuit board without having cumbersome assemblies attached to board edges. This latter requirement permits easy removal and substitution of boards in the chassis without first having to provide a specialized assembly at the end of the circuit board.